The present invention relates to method and apparatus for using a particular type of tubing for chemical analysis, preferably as a fiber suppressor in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a first chromagraphic separation stage, a second suppression stage, followed by detection, typically by an ion conductivity detector. In the first stage ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the second stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,296,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin column. A different form of suppressor column is described in published European patent application No. 32,770 in which a charged membrane, typically in the form of a fiber, is used in place of the resin bed. The sample and eluent are passed through the fiber with a flowing regenerant at the outside wall of the fiber. The fiber comprises an ion exchange membrane partitioning the regenerant from the effluent of chromagraphic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form followed by detection of the ions.
One problem with a hollow fiber suppressor system is sample band spreading which degrades the chromatographic resolution. That is, where laminar flow exists, as by the passage of a viscous liquid through a smooth walled fiber, the band of sample broadens or spreads as it flows through the fiber. One way to reduce band spreading is to disrupt the laminar flow by packing of the fiber with beads as described in Stevens, T. S., et al, Anal. Chem. 1982, 54, 1206.
One problem with packing of the fiber is that it is labor intensive. Another problem is that the fiber swells or expands under pressure or in contact with solvent during use. This increases the inner diameter of the fiber and permits the beads to concentrate at the outlet end of the fiber. This leaves gaps along the length of the fiber in which there are little or no beads where band spreading can occur. It would be desirable to provide an improved technique for minimizing band spreading which is not as labor intensive as packing of the fibers would be and would eliminate band spreading due to movement of the beads during use.